In a work area wherein production processes are controlled, large numbers of instruments (for example, sensor instruments and devices such as valve positioners) are located throughout the plant in order to control the processes. The various instruments within the plant are typically connected in a hierarchical structure. Consequently, in a system for controlling instruments the instruments are displayed in a control screen in a tree format matching that hierarchical structure, to enable the large number of instruments, which exist in a complex arrangement, to be understood easily. Japanese Unexamined Patent Application Publication 2005-346444 (“JP '444”) discloses a control system for displaying, using icons, alert statuses that are produced by the individual instruments, with the instruments displayed in a tree format.
However, for example, in performing maintenance or inspections on the individual instruments, or when replacing the instruments, the instruments are temporarily cut off (disconnected) from the system when the operation is performed. Consequently, when, for example, an instrument that is on a higher hierarchical level is disconnected, the group of instruments that are located hierarchically therebelow are also cut off from the system. However, because the group of instruments located on the lower hierarchical levels continue to operate even when they are cut off, it is possible that alerts may be produced while they are cut off. In the control system of JP '444, above, the alert status at the current point in time is displayed for each of the instrument status connected to the system, but no thought is given to those instruments that are cut off from the system. Consequently, in this control system, it is not possible to ascertain, on the control screen, alert statuses that have been produced in the group of instruments on the lower hierarchical level during the time period in which they are cut off from the system when an instrument on a higher hierarchical level is temporarily disconnected and then connected again thereafter.
The present invention was created in order to solve the problem area set forth above in the conventional technology, and the object thereof is to provide an instrument status displaying device and instrument status displaying method able to display also those statuses of instruments for the period of time over which they were cut off from the system.